Title basically sums it up... Every time I open my MC launcher and press play on Java, the Launcher requires me to enter a username, every... single... time..., despite already having a name, and a profile, under my MS account, which obvious I have to be using...

The issue seems to be rectifiable... after you sign-out and sign back in:

Under Settings in the bottom-left corner, go to Accounts, then click the 3 dots next to your account name and click "Remove account".

Then sign back in...